Prepare cooked vegetables:
Have ready a large bowl of ice and cold water. Cook beans in a large pot of boiling salted water until crisp-tender, about 4 minutes. Transfer beans to ice water with a slotted spoon to cool, then transfer to a colander to drain.

Add cauliflower to boiling water and cook until crisp-tender, about 4 minutes. Transfer to ice water to cool, then transfer to colander to drain.

Add turnips to boiling water and cook until crisp-tender, about 7 minutes. Transfer to ice water to cool, then drain in colander.
Cook potatoes about 30 minutes.

Before serving:
Steam potatoes in a large steamer over boiling water, covered, until tender, 10 to 15 minutes. Serve hot or warm.